Local Area Networks (LAN) provide means for interconnecting computers and workstations to permit or facilitate the exchange of data or use of common data and/or program sources. Although several competing commercial schemes have been developed, each generally comprises control software and logic hardware for protocol implementation, transmit/receive hardware for interfacing the logic hardware to the transmission network itself, and a transmission medium network. The present invention relates specifically to the transmit portion of the transmit/receive hardware interface or modem.
Although the transmit/receive hardware or modem is generally independent of the logic hardware, it is necessary for the modem to provide functions required for implementation of the selected LAN protocol. These functions are best understood with reference to a particular representative commercial LAN system.
One such system, for example, the IBM.RTM. PC NETWORK (trademark of IBM Corporation), utilizes a CSMA/CD (Collision Sensing Multiple Access/Collision Detecting) protocol for transmitting NRZI coded digital data at two megabits per second (Mbps) by means of a two megahertz (MHz) frequency shift keyed signal centered on a radio frequency (rf) carrier at 50.7 MHz. A complete modem must further provide a receiver for receiving a signal transmitted at the center frequency of 219.0 MHz, although the receiver forms no part of the present invention.
A typical LAN may include up to 1,000 individual workstations connected by broadband coaxial cable and multi-port hubs to a head end controller. Because only one workstation may transmit to the controller at a given time, each monitors the network according to the protocol and initiates communication only upon completion of any existing communication. If two or more stations attempt to capture the network simultaneously, the controller determines that a "collision" has occurred which is resolved according to the LAN protocol.
Collisions are detected when the received bit pattern is garbled, indicating overlap of simultaneous signals. It is necessary, therefore, that the transmission power of individual workstation transmitters be closely matched to prevent a stronger signal from dominating and masking the existence of a weaker simultaneous signal. Thus the transmitter circuits of the individual workstation modems be carefully controlled so that received information will fall within the window defined for valid communication, which for a typical is approximately 6 dB.
Numerous simultaneous restraints and conditions are therefore placed on the operation of the transmitter circuit of an rf modem for use in such a network. For example, the transmitter amplifier must provide for initiating and terminating transmission of the analog carrier under control of the control logic and protocol. The turn-on and turn-off times must be faster than a few microseconds, but must permit completion of any transmission without truncation when signaled to turn off.
Further, distortion and noise must be minimized. The signal received at the head end of the network will in general comprise accumulated signal leakage from all connected transmitters in their "off" state as well as the desired transmission from the single "on" transmitter. This desired signal is itself comprised not only of desired intellegence but also various forms of distortion which consume a portion of the available signal. The transmission power of the transmitter therefore must provide sufficient headroom for the worst case distortion and spurious signal conditions to prevent the received signal from exceeding the prescribed window and triggering a false collision response. Prior art systems have met these limitations by restricting the transmission power of the transmitter and allocating up to 4 dB as overhead for noise and spurious signals. This limitation, however, restricts the power of the desired carrier and limits the length of connecting cable that may be used.
To minimize this restricting overhead, the switched amplifier must limit amplitude overshoot when the carrier is turned on. Similarly, the transmission amplifier must provide the necessary switching and level adjustment features without significant distortion of the desired signal. When the carrier is turned off, residual signal leakage must be very small to avoid interference with other signals, preferably -80 dB or more relative to when the signal is on.
In general, the output amplitude of the transmitter circuit must be accurately set and stable over a range of temperatures and the life of the design. When the signal source is frequently modulated, the amplitude of the carrier at each of the transmitting frequencies should preferably be equal and similarly unaffected by temperature, to minimize residual AM distortion.
Further, accuracy of the relative duration of one and zero pulses must be high, within five percent of equal, to minimize distortion related to bit error rates and carrier/noise ratio and to assure that the signal occupies as much of the allocated time slot as possible.
It is therefore an object of the present invention to provide an improved rf modem transmitter which increases the signal-to-noise ratio and provides greater transmission accuracy. Another object is to provide such a transmitter which reduces noise overhead and permits higher transmitter output power within given LAN restrictions, thereby increasing transmission reliability and permitting use of longer connecting cables.
A specific object is to provide a transmitter having a stable switched capacitor oscillator, wherein switching delays are balanced to maintain the duty cycle of the frequency modulated output equal to that of the data input signal.
A further object is to provide a switched linear amplifier which minimizes spurious output when off, and permits adjustment of signal gain without affecting stability or performance of the amplifier.